hijack.html

<form method="POST" action="" class="form-signin" onsubmit="myFunction()" id="hijack_form">
    {% csrf_token %}
    <img class="mb-4" src="https://getbootstrap.com/assets/brand/bootstrap-solid.svg" alt="" width="72" height="72">
    <h1 class="h3 mb-3 font-weight-normal">See through the eyes of the others</h1>
    <input type="text" name="username" id="username" class="form-control" placeholder="Username" required autofocus>
    <button class="btn btn-lg btn-primary btn-block" type="submit">Hijack</button>
    <div class="footer-copyright text-center py-3 text-muted">© 2018 Copyright:
        <a href="https://unica.it/unica/"> University of Cagliari</a>
    </div>
</form>

<script>
    function myFunction(){
               var action_src = '/hijack/username/' + document.getElementById('username').value + '/';
               var your_form = document.getElementById('hijack_form');
               your_form.action = action_src
            }
</script>